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/*
 * heap.h -- Shared-memory binary min-heap (priority queue) for Linux
 *
 * Mutex-protected push/pop with sift-up/sift-down.
 * Futex blocking when empty (pop_wait).
 * Elements are (int64_t priority, int64_t value) pairs.
 * Lowest priority pops first (min-heap).
 */

#ifndef HEAP_H
#define HEAP_H

#include <stdint.h>
#include <stdbool.h>
#include <stdlib.h>
#include <string.h>
#include <unistd.h>
#include <fcntl.h>
#include <errno.h>
#include <time.h>
#include <limits.h>
#include <signal.h>
#include <sys/mman.h>
#include <sys/stat.h>
#include <sys/file.h>
#include <sys/syscall.h>
#include <linux/futex.h>
#include <sys/eventfd.h>

#define HEAP_MAGIC       0x48455031U  /* "HEP1" */
#define HEAP_VERSION     1
#define HEAP_ERR_BUFLEN  256
#define HEAP_SPIN_LIMIT  32

#define HEAP_MUTEX_BIT   0x80000000U
#define HEAP_MUTEX_PID   0x7FFFFFFFU

/* Indices into the heap data are uint32_t (sift_up/down).  Cap capacity
 * to UINT32_MAX so size++ in heap_push cannot wrap and the (uint32_t)
 * cast against hdr->capacity in the capacity check cannot truncate. */
#define HEAP_MAX_CAPACITY ((uint64_t)UINT32_MAX)

typedef struct {
    int64_t priority;
    int64_t value;
} HeapEntry;

/* ================================================================
 * Header (128 bytes)
 * ================================================================ */

typedef struct {
    uint32_t magic;
    uint32_t version;
    uint64_t capacity;
    uint64_t total_size;
    uint64_t data_off;
    uint8_t  _pad0[32];

    uint32_t size;             /* 64: current element count (futex word for pop) */
    uint32_t mutex;            /* 68: 0=free, HEAP_MUTEX_BIT|pid=locked */
    uint32_t mutex_waiters;    /* 72 */
    uint32_t waiters_pop;      /* 76 */
    uint64_t stat_pushes;      /* 80 */
    uint64_t stat_pops;        /* 88 */
    uint64_t stat_waits;       /* 96 */
    uint64_t stat_timeouts;    /* 104 */
    uint64_t stat_recoveries;  /* 112 */
    uint8_t  _pad1[8];         /* 120-127 */
} HeapHeader;

#if defined(__STDC_VERSION__) && __STDC_VERSION__ >= 201112L
_Static_assert(sizeof(HeapHeader) == 128, "HeapHeader must be 128 bytes");
#endif

typedef struct {
    HeapHeader *hdr;
    HeapEntry  *data;
    size_t      mmap_size;
    char       *path;
    int         notify_fd;
    int         backing_fd;
} HeapHandle;

/* ================================================================
 * Mutex (PID-based, stale-recoverable)
 * ================================================================ */

static const struct timespec heap_lock_timeout = { 2, 0 };

static inline int heap_pid_alive(uint32_t pid) {
    if (pid == 0) return 1;
    return !(kill((pid_t)pid, 0) == -1 && errno == ESRCH);
}

static inline void heap_mutex_lock(HeapHeader *hdr) {
    uint32_t mypid = HEAP_MUTEX_BIT | ((uint32_t)getpid() & HEAP_MUTEX_PID);
    for (int spin = 0; ; spin++) {
        uint32_t expected = 0;
        if (__atomic_compare_exchange_n(&hdr->mutex, &expected, mypid,
                1, __ATOMIC_ACQUIRE, __ATOMIC_RELAXED))
            return;
        if (spin < HEAP_SPIN_LIMIT) {
#if defined(__x86_64__) || defined(__i386__)
            __asm__ volatile("pause" ::: "memory");
#elif defined(__aarch64__)
            __asm__ volatile("yield" ::: "memory");
#endif
            continue;
        }
        __atomic_add_fetch(&hdr->mutex_waiters, 1, __ATOMIC_RELAXED);
        uint32_t cur = __atomic_load_n(&hdr->mutex, __ATOMIC_RELAXED);
        if (cur != 0) {
            long rc = syscall(SYS_futex, &hdr->mutex, FUTEX_WAIT, cur,
                              &heap_lock_timeout, NULL, 0);
            if (rc == -1 && errno == ETIMEDOUT && cur >= HEAP_MUTEX_BIT) {
                uint32_t pid = cur & HEAP_MUTEX_PID;
                if (!heap_pid_alive(pid)) {
                    if (__atomic_compare_exchange_n(&hdr->mutex, &cur, 0,
                            0, __ATOMIC_ACQ_REL, __ATOMIC_RELAXED)) {
                        __atomic_add_fetch(&hdr->stat_recoveries, 1, __ATOMIC_RELAXED);